Method of forming an investment mold with potassium sulfate additive



United States Patent 3,234,607 METHOD OF FORMING AN INVESTMENT MOLD WITHPOTASSEUM SULFATE ADDITIVE Bengt G. Hammarlund, Alvsjo, Sweden, assignorto Bengt B. Edholm, Redondo Beach, Calif. N0 Drawing. Filed May 15,1963, Ser. No. 280,704 4 Claims. (Cl. 22196) This invention relates toan investment composition and method of making an investment casting.

The composition of this invention and the novel method described isparticularly utilized for the manufacture of refactory molds for castingmetals such as gold alloy or other suitable casting material into inlaysor other objects where the article cast must conform as precisely aspossible in detail to the size and shape required, or the invention maybe utilized wherever a precise investment casting is desired. It isparticularly useful in a primary concern in the field of the manufactureof dental restorations to provide a suitable restoration for a tooththat has been damaged. A dentist normally proceeds to suitably shape thedamaged tooth and then takes an impression thereof in a plastic materialto provide a working model. On this model, a wax pattern is made whichprovides an exact replica of the required restoration. The wax patternis used to form a mold which in turn is utilized to make a casting of asuitable metal alloy. This casting is used as the restoration. It isobvious that it is particularly desirable that the casting so providedfit the damaged tooth as precisely as possible and conformdimensi0n-wise to the tooth previous to damage thereof.

The procedure for making the precise metal alloy casting consists offirst applying a warm wax to the working model. A small rod or sprue isinserted into the wax and then after chilling the wax pattern is removedby means of this sprue. The mold for the alloy is then formed bycarefully placing a plastic material around the pattern. This materialis generally known as the investment. This investment is then allowed toharden. The investment is then heated to burn out the Wax with the spruehaving been removed. The investment is heated up to a temperaturesuitable for casting. And the metal in molten form is forced into theinvestment and allowed to cool forming the final restoration.

The metal normally used in most castings is a gold alloy which undergoeschanges in volume with the change in temperature when passing eitherfrom the solid or liquid state.

The investment materials normally used comprise a binder material suchas plaster of Paris and a filler matefial with various other inert ormodified materials. The filler material is generally a form of silicasuch as ground silex and cristobalite, tridymite and vitreous silica allhaving the same chemical composition but differing widely in suchproperties as crystalline form, optical index of refraction, density orspecific gravity and thermal expansion. The particular outstandingsource for the above materials for making investments is ground siliciabrick.

Prior to this invention the mold or investment was made, as previouslymentioned, by forming the plastic material through mixing the binder andsilica with water and utilizing this mixture to surround the waxcasting. The mold is then allowed to set for a period of usually about22 minutes in a water bath of about 104 F. This mold is then removedfrom the water and put into an oven which is initially at roomtemperature at the time the mold is inserted. The temperature is thenraised gradually to 900 F.-1300 F. over approximately a two hour period.Previous experience has shown that it is essential to the production ofa good mold to gradually raise the temperature of the oven to 1300 F. toavoid destruction of the mold by cracking or total disintegration. Thus,it can readily be appreciated, the necessity 3,234,607 Patented Feb. 15,1966 for allowing the oven to gradually raise the temperature overapproximately a two hour period, presents numerous problems to theindividual preparing the investment. A given oven is not readilyavailable throughout the working day for continuous use for it must beraised in temperature, as previously set forth, for a two hour periodand then allowed to cool down again to room temperature before it isready for use for another preparation for investment. As a result ofthis, the individual preparing the investments must plan his work sothat he may insert several investments into the oven at one time, so asto most effectively utilize its capacity. Alternatively of course,numerous ovens may be utilized. However, the expense of doing so and ofcontrolling this number is not a desirable solution. It should also beunderstood that the control of the rise in time to the desiredtemperature is critical and provides a possible stumbling block to thepreparation of successful investments.

It is therefore an object of this invention to provide an investmentcomposition and method of casting in which the residence time of thecasting within the oven utilized is considerably less than thatpreviously known.

It is another object of this invention to provide an investment castingmethod in which no gradual raising of the temperature of the ovenutilized is required.

A still further object of this invention is to provide an investmentcomposition and method of casting which provides for greater utilizationof the oven needed in the process.

One further object of the invention is to provide an investmentcomposition and casting method requiring a lower oven temperature andoven residence time than in previous methods and for previouscompositions.

The above and other objects will become apparent from the followingdetailed description.

The investment composition of this invention comprises a silicamaterial, a binder, and potassium sulfate solution sufiicient to make apaste suitable for casting. It has been discovered that the substitutionof a potassium sulfate solution for the previously utilized water toform the casting base provides for the highly desirable properties ofthe novel investment composition described. The method for thepreparation of a suitable investment utilizing this novel compositionembraces a residence time of approximately 20 minutes in an ovenmaintained at about 1300 F., substantially reducing the set timepreviously known in the art.

It has been discovered that the potassium sulfate solution utilizedshould be at a particular concentration to accomplish the desiredresults. Thus, the process comprises the obtaining of such a solution.In order to do this, the potassium sulfate is dissolved in water,preferably distilled water, in a ratio of one gram of sulfate to 10 cc.of the water. The solution formed is shaken for a period of time to besure that all of the sulfate has gone into solution. When it appearsthat the sulfate has completely gone into solution this solution is thendiluted in the ratio of one part of the sulfate solution to nine partsof distilled water, for use in mixing with the solid other materials toform the paste for the investment. The solution of the potassiumsulfate, is then mixed with the It has been found that l04 F. is a.particularly preferred temperature for this water bath. The materialgenerally will not set up at temperatures in excess of 104 While attemperatures below this figure, one does not obtain the full expansionof the mold which is preferable for the desired results. Generally theperiod of time for the material to be submerged in the water bath rangesfrom two to seven minutes with the range from five to seven minutesbeing preferred. While successful investments have been made when theresidence time in the bath is less than five minutes, these results havenot been entirely consistent thus this preferred range of five to sevenminutes is given.

After the material has set in the water bath, it is generally moved andplaced in an oven which has a temperature not exceeding 1300 F. Theinvestment is kept in the oven until it turns a dark red color which isdue to a glow from the absorbed heat therein. This generally occurs inabout fifteen to twenty minutes at the 1300 F. Though temperatures below1300 can be utilized, such as for example 1100 F., the process wouldtake considerably longer to transpire, thus it is not preferable thoughsuch may be accomplished. However, at temperatures much in excess of1300 F. a sufii-cient investment cannot be successfully obtained becauseof improper expansion of the mold and possible breakdown of theinvestment material. The investment is then removed from the oven andthe mold obtained is then utilized to form the restoration in aconventional manner.

As can now be seen from the novel composition and method of investmentcasting, the residence time of the mold in the water bath has beenreduced from the state of the art 22 minutes to from 5 to 7 minutes.This water bath plays an important role in the process of preparing aninvestment casting in that it gives the neces sary preliminary expansionto the investment. This expansion is also known as hygroscopicexpansion. Additionally, as can now be seen, through the utilization ofthe novel composition the casting residence time in the oven has beenreduced from a previous two hours to approximately 15 to 20 minutes. Theimportant factor of removing the need for slowly raising the temperaturein the oven is apparent. As a result the oven or ovens utilized by anindividual engaged in the practice of preparing such investments may allbe maintained at 1300 F. constantly; thus the production of suchinvestments may be continuous throughout the working day or week. Nolonger are so called batch processes for the preparation of theseinvestments needed. This should substantially increase the capacity ofone engaged in this practice.

To specifically illustrate by way of example, an investment preparedaccording to this invention, one gram of potassium sulfate was placed in10 cc. of distilled Water in a normal test tube which was subjected toshaking until the potassium sulfate granules had disappeared andcompletely gone into solution; this solution was then diluted bydissolving 10 c. thereof in 9 cc. of distilled water. The solution wasthen added to a mixture comprised of 15 grams of plaster of Paris, 25grams of cristobalite and 10 grams of tridymite. 15 cc. of the solutionwas stirred with the powder in a glass mixer until a smooth pasteconsistency was obtained. This paste was then placed around a waxpattern so as to obtain a suitable mold for casting. The paste and theWax was then completely submerged from to 7 minutes in a water bathmaintained at 104 F. Upon removal from the bath at the given time thematerial was then placed in an oven which had a temperature of 1300 Thematerial was kept in the oven for 15 to 20 minutes at which time itturned a dark glowing red. After removal from the oven the investmentwas ready for utilization to form a restoration.

The potassium sulfate. solution of the invention may be utilized withvarious different binders as previously described, the binders notforming a novel part of the invention. In addition to the plaster ofParis previously set forth, other binders may be Keenes cement,Portlandv 4 cement, oxy salts cements, magnesium oxide cements,phosphoric acid cements, sodium silicate cements, and the like. Though,generally; plaster of Paris is to be preferred as the binder material.Usually the binder is present in amounts over 20% by weight of thepowder composition.

While this invention has been described and illustrated in detail, it isto be clearly understood that this is intended by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the invention being limited only by the terms of thefollowing claims.

I claim:

1. A method of preparing an investment casting of a pattern comprisingforming an aqueous solution of potassium sulfate in the proportion of 1gram of sulfate to 10 cc. of water,

diluting said aqueous sulfate solution by adding 9 parts of water to 1part of said solution,

mixing silica and a binder which will set in the presence of Warm waterwith sufficient potassium sulfate solution to create a paste,

forming said paste about said pattern to provide said investment,

providing a water bath,

heating said bath to a temperature sufficient to cause said investmentto set when submerged therein, submerging said investment in said bath,

maintaining said investment in said bath for a suflicient period of timeto cause said investment to preset, removing said investment from saidbath,

providing an oven,

heating said oven to a temperature sutficient to cause said investmentto completely set,

placing said investment in said oven,

removing said investment from said oven when said investment turns to adark red color.

2. The method of preparing an investment casting comprising forming anaqueous solution of potassium sulfate in the proportion of 1 gram ofsulfate to 10 cc. of water,

diluting said aqueous sulfate solution by adding 9 parts of water to 1part of said solution, mixing silica and a binder which will set in thepresence of warm water with a sufficient amount of said dilutedpotassium sulfate solution to create a paste,

forming said paste about a pattern providing an investment,

submerging said investment in a water bath heated to a temperaturesufficient to provide presetting of said investment,

subsequently placing said investment in a heated oven until saidinvestment turns a dark red color.

3. The method of claim 2 wherein said water bath is maintained at atemperature not exceeding 104 F.

4. The method of claim 2 wherein said oven is maintained at atemperature not exceeding 1300 F.

References Cited by the Examiner UNITED STATES PATENTS 1,932,202 10/1933Coleman et al 106-383 2,054,894 9/1936 Busch et al. 22l90.1 2,391,85512/1945 Bean 22-193 2,513,212 6/1950 Saives 22193 2,529,835 11/1950Dailey et al 22-193 FOREIGN PATENTS 164,669 10/1933 Germany.

OTHER REFERENCES Mellor, Modern Inorganic Chemistry, published 1939, pp.15 O1 5 3.

MARCUS U. LYONS, Primary Examiner.

1. A METHOD OF PREPARING AN INVESTMENT CASTING OF A PATTERN COMPRISINGFORMING AN AQUEOUS SOLUTION OF POTASSIUM SULFATE IN THE PROPORTION OF 1GRAM OF SULFATE TO 10 CC. OF WATER, DILUTING SAID AQUEOUS SULFATESOLUTION BY ADDING 9 PARTS OF WATER TO 1 PART OF SAID SOLUTION, MIXINGSILICA AND A BINDER WHICH WILL SET IN THE PRESENCE OF WARM WATER WITHSUFFICIENT POTASSIUM SULFATE SOLUTION TO CREATE A PASTE, FORMING SAIDPASTE ABOUT SAID PATTERN TO PROVIDE SAID INVESTMENT, PROVIDING A WATERBATH, HEATING SAID BATH TO A TEMPERATURE SUFFICIENT TO CAUSE SAIDINVESTMENT TO SET WHEN SUBMERGEN THEREIN, SUBMERGING SAID INVESTMENT INSAID BATH, MAINTAINING SAID INVESTMENT IN SAID BATH FOR A SUFFICIENTPERIOD OF TIME TO CAUSE SAID INVESTMENT TO PRESET, REMOVING SAIDINVESTMENT FROM SAID BATH PROVIDING AN OVEN, HEATING SAID OVEN TO ATEMPERATURE SUFFICIENT TO CAUSE SAID INVESTMENT TO COMPLETELY SET,PLACING SAID INVESTMENT IN SAID OVEN, REMOVING SAID INVESTMENT FROM SAIDOVEN WHEN SAID INVESTMENT TURNS TO A DARK RED COLOR.